A question of fundamental importance for all ages is how the overabundance of information in the environment is selectively filtered to allow for coherent information processing. The answer is central to the development of cognition, its efficiency and its behavioral manifestation. To answer this, we rely on the construct of selective attention, described as the process in which the mind takes hold of one out of a multitude of possible objects while ignoring the others. Selective attention mechanisms are important because processing resources are limited making processing the immense amount of information in the visual world all at once difficult. For the infant, who is constantly assailed by a multitude of new information and whose processing resources are less efficient and more limited than adults, being capable of selectively attending would seem to be crucial to constructing an organized and detailed representation of their world. This is supported by the fact that certain developmental disabilities, such as Attention Deficit Disorder and Autism, are characterized by deficits in the ability to selectively allocate attention. Research is therefore proposed to begin to address the gaps in our understanding of the development of selective attention mechanisms by systematically investigating with an eye movement reaction time measure that is more appropriate for evaluating the selection mechanisms and more similar to the manner in which adults'selective attention mechanisms are typically measured. Moreover, by measuring eye movements, both infants and adults can be tested in the same paradigm with the collection of the same qualitative data, enabling a more direct comparison between the selective attention mechanisms of infants and adults and their developmental characteristics. In adults, a number of different mechanisms of attentional selectivity have been investigated, in particular, preattentive versus attentive mechanisms, mechanisms for selecting a target from among other potential targets, and mechanisms of inhibiting processing (i.e. ignoring) of non-selected information. None of these mechanisms have received much or any focus in developmental research. The proposed research will therefore focus on the three selective attention mechanisms and it is expected that the substance of infants'manifestation of these mechanisms will not be different than adults but that the infants'processing will be slowed relative to adults. The proposed research has the potential to, for the first time, answer some long-standing questions about the nature of infants'ability to make sense of their worlds. In the future, the proposed research will also enable a better understanding of what goes wrong in development when infants and children have trouble making sense of their worlds. Understanding these mechanisms is critical to getting a handle on brain development and potential developmental disabilities such as Attention Deficit Disorder and Autism. PUBLIC HEALTH RELEVANCE: A question of fundamental importance for cognitive development is how infants, with limited cognitive resources, are able to selectively filter the overabundance of information in their environment to allow for coherent learning about their world. The particular objective of this research proposal, therefore, is to begin to delineate the mechanisms of selective attention in early development by use of a new methodology of assessing infants'eye movements. The proposed research also has the potential to enable a better understanding of what goes wrong in attentional development when infants and children have trouble selectively filtering the information in their worlds, such as in Attention Deficit Disorder and Autism.